Various intramedullary nails and targeting devices for interlocking the intramedullary nail to the surrounding bone, particularly for the use in repairing the femur, are known in the prior art. One targeting method that is capable of providing precise locating of the holes distally uses x-ray techniques, but long periods of x-ray exposure are required and the need to move the x-ray equipment in and out of position to check the screw or pin locations means that there is a risk of a loss of alignment each time the equipment is moved. Patents of interest in this field include U.S. Pat. Nos. 5,537,453 (Williams et.al.); 5,478,343 (Ritter); 5,426,687 (Goodall et al); 5,178,621 (Cook, et. al.); 5,031,203 (Trecha); 5,030,222 (Calandruccio et al); 5,013,317 (Cole et al); and others as cited in the above patents. As a consequence of these radiographic techniques, the positioning of such locking screws or pins is typically the most time consuming and difficult portion of the overall rod implantation procedure.
Two other patents are thought to be of more general interest, U.S. Pat. Nos. 4,625,718 (Olerud et al.), and 4,570,624 (Wu). The Olerud et al. patent disclosing an aiming apparatus using X-ray techniques for making holes or bores in the bone of a patient in registration with the holes or bores on an interlocking nail, and the Wu patent disclosing a mechanical technique for aligning surgical pins in parallel.
Patents of interest in this field include U.S. Pat. Nos. 4,621,628 (Brudermann); 5,049,151 (Durham et al); and 5,514,145 (Durham et al). The Brudermann patent discloses an apparatus for locating transverse holes in the distal end of implanted locking nails. The apparatus includes at least one magnet which generates an axially symmetrical field, in combination with a magnetic field detecting device or sensor having an axial field reception characteristic. In one embodiment, the magnetic field sensor is inserted into an implanted nail and the magnet, which is placed on the surface of the skin, is moved until axes of the magnetic field of the magnet and the sensor are aligned. More particularly, the sensor is connected to an external display device and alignment of the respective magnetic fields is indicated when a zero-point indication is provided on the display device. A second magnet can be used to increase the precision of the alignment process. The directional characteristics of the magnetic field detection device are used to control the relative positions of the axes of both directional elements through a display device, such that both axes are brought into congruence with each other by means of the control display. When one of the directional elements is aligned exactly with the axis of the of the transverse hole in the in the nail, another element can be used externally to mark the location of the nail hole for positioning of a drilling jig.
The two patents by Durham et al. relate to a method and apparatus for positioning the screws or pins of orthopedic hardware devices such as intramedullary rods which involves the positioning of a first magnet at the location of a screw hole in the nail and then using an aiming device, comprising a second magnet which interacts with the first magnet, to locate the first magnet and hence enable a screw or pin to be placed in the screw hole in the nail to lock the nail in position.
In one first embodiment, an insertion rod is used to position the first magnet at the level of the hole in the rod while in another embodiment, a solid nail is used and the magnet is removable disposed within the hole in the nail prior to implantation of the nail.
One serious disadvantage common to the magnetic field detection devices is the detrimental influence of stray magnetic fields, such as, for example the earth magnetic field, or the effect of field distortion due to highly conductive materials in the form of aluminum, titanium, stainless steel and copper used in the construction of operating room tables and surgical implants. The art of using transmitting and receiving components with electromagnetic coupling for measuring position and orientation is well known especially with respect to armament sighting systems where the receiver component would be located in a gunner's helmet and a transmitter component would be attached to a nearby electrically non-conductive structure. As the gunner would sight-in a target through a sighting cross-hair affixed to his helmet, the receiver located thereupon would pick up signals generated by the transmitter. These signals would then be processed by a computer to determine the position and orientation of the helmet and then to contemporaneously point a unit of armament in the same direction as the helmet mounted sight piece. As taught in U.S. Pat. No. 4,054,881 (Raab) and U.S. Pat. No. 4,287,809 (Egli et al.), and U.S. Pat. No. 4,314,251 (Raab) and U.S. Pat. No. 4,396,885 (Constant), an alternating current (AC) signal is applied in a time division or frequency division format to a transmitter consisting of two or three orthogonal coils which generate an AC electromagnetic field which is measured by an AC receiver likewise consisting of three or two orthogonal coils. These sensed signals are then filtered and amplified in a method compatible with the transmitted format, converted to a digital format and then read into a computer where various mathematical methods are resorted to in order to extract position and orientation with resort to applicable electromagnetic field equations.
All current systems such as the ones above, that utilize an AC transmitted signal work accurately only when there are no electrically conductive materials located near either the transmitter or receiver because any transmitted AC signal would invariably induce eddy currents in these conductive materials which would in turn serve to generate an AC magnetic field that would distort any transmitted field, and, of course, any ultimate output position and orientation data. In fighter aircraft or helicopters where it is desired to use these position and orientation measuring systems, there are a lot of highly conductive materials in the form of aluminum, titanium, magnesium, stainless steel, and copper used in the construction of the cockpit structure, seat, wiring and helmet-mounted displays. U.S. Pat. No. 4,287,809 teaches a method of compensating for the errors resulting from any field distortion due to cockpit metal that does not move with respect to the transmitter. The compensation method therein suggested involves making measurements throughout the cockpit to determine the amount of such distortion and then using this data to form a correction that is applied to the sensed signals. In a similar manner, U.S. Pat. Nos. 4,394,831 (Egli et al.) and 4,621,628 (Brudermann) teaches a method to accomplish compensation for errors due to eddy currents induced in metal such as would be found in a display located on a pilot's helmet or operating ing field, respectively. This compensation method again requires initial experimental measurements of such distortion in order to effect necessary corrections and provides moderate improvements in accuracy only when the amount of metal is concentrated in a single location and the transmitter does not go through large angular rotations or translations. These types of compensation efforts that are required to make AC systems work accurately are time consuming and expensive to perform and only work in environments where there would not be too much conductive material near transmitter or receiver units. In many locations, for example, AC systems cannot be utilized at all because the distortions produced are simply too large to be corrected merely by such mapping.
It is the object of this invention to provide an effective and economical device for the determination of the location and orientation of the holes in orthopaedic implants. Still another object of the present invention is to provide a targeting device which can be utilized by the majority of current intramedullary nails currently available to the surgeon.